Controls for the automatic operation of storage and retrieval machines which are presently used are typically based on a single control circuit which controls all three operating axes, that is, the base, hoist carriage, and shuttle travel, through an open loop or a sloppy closed loop feedback circuit. The single control handles all of the positioning information for the three different operating axes. To move the base, carriage and shuttle of the storage and retrieval machine from one location to another, the current position is monitored and that position is compared with the new position. The control then supplies appropriate direction and speed reference signals to motor drives to make all three movements. While the base, carriage and shuttle move to their desired positions, the control continuously monitors the current position of each. As each device approaches its new position, the control decreases the speed reference signal which, in turn, decreases the motor speed, when the device reaches the desired destination.
As a result of controlling the base, carriage and shuttle devices simultaneously on their three separate axis, the control is overworked resulting in delay and inaccuracy in the operation of the storage and retrieval machine.
Other types of control problems in presently used storage and retrieval machines include poor positioning accuracy due to the use of standard industrial motors having high rotor inertias and which are not effectively controllable in a closed loop system. Also, present positioning feedback systems utilize photodiode arrays and coded plates or code bars which provide position information only every few feet and have poor resolution.